1. Field of the Invention
This invention relates to a micro-coaxial wiring structure fabricated in VLSI dimensions, as well as to a manufacturing method for making such a structure.
2. Discussion of the Related Art
Improvements in the VLSI technology are resulting in smaller interconnect and spacing, larger chip sizes, increased circuit density, and faster devices. The average and the maximum on-chip interconnect lengths are becoming larger because of the increase in chip size as well as device density. Owing to the larger interconnect length and faster devices, i.e., faster signal rise times, the wires connecting the devices on the chip act like transmission lines. In other words, the interconnect wire can, no longer, be considered just a lumped capacitive load, rather, it becomes a linear network of resistance (R), inductance (L), and capacitance (C) elements. Thus, the interconnect needs to be appropriately modeled to optimize the high frequency pulse propagation without severe signal degradation and losses. Two of the transmission line parameters, i.e., the characteristic impedance (ZO), and the attenuation constant (.alpha.), serve as metrics for the high frequency pulse propagation.
The increased wiring density reduces the spacing between adjacent signal lines resulting in increased coupled noise. The coupled noise is an unwanted electromagnetic interference in the signal pulse degrading the signal fidelity. The backward coupling coefficient (Kb) characterizes the near-end noise (NEN) between two adjacent signal lines.
The faster rise times and broader signal bandwidth are making it imperative that interconnects be modeled as transmission lines and that the coupled noise be held to a minimum. Thus, there is a need to design the on-chip interconnects having a controlled electromagnetic environment for the propagation of high speed signals.
In U.S. Pat. No. 4,776,087, issued Oct. 11, 1988 to Cronin et. al., a VLSI coaxial wiring structure is disclosed which fully shields the signal conductor for eliminating the coupling effects from adjacent signal conductors. The fully shielded structure of Cronin et. al., however, is not an optimum structure from the perspective of high speed pulse propagation. To minimize noise in the signal lines, the characteristic impedance of the signal lines should be as close to 50 ohm as possible. As the impedance is dependent upon the geometry of the structure, the structure taught by Cronin et. al. provides for a low characteristic impedance because of increased capacitive coupling due to the complete shielding. The complete shielding results in a higher overall noise, since only a ground connection is provided for. This further results in increased pulse attenuation for the rising transition of a propagated signal and, still further, results in increasing the simultaneously switching noise due to the higher power supply inductance.
There is thus a need for a micro-coaxial wiring structure which provides signal lines having a characteristic impedance as close to 50 ohm as possible and which improves high speed pulse propagation while substantially reducing cross-talk between adjacent signal lines.